Petroleum refiners often produce desirable products such as diesel fuel, naphtha, and gasoline, by hydrocracking a hydrocarbon feedstock, normally derived from crude oil. Feedstocks, typically distillate feedstocks often subjected to hydrocracking are gas oils and heavy gas oils recovered from crude oil by distillation. For example, U.S. Pat. No. 4,943,366 discloses a hydrocracking process for converting highly aromatic, substantially dealkylated feedstock into high octane gasoline.
Refiners also subject distillate hydrocarbon streams to hydrotreating operations such as hydrodesulfurization and hydrodenitrogenation. In hydrodesulfurization, sulfur atoms are removed and typically converted into hydrogen sulfide. Similarly, in hydrodenitrogenation, nitrogen atoms are removed from the hydrocarbon and converted into ammonia. As discussed in U.S. Pat. No. 8,066,867, there are some benefits achieved by hydrotreating the feedstock before subjecting it to hydrocracking.
In such processes, in which a feed is first hydrotreated and then subjected to hydrocracking, hydrogen sulfide and ammonia may be passed to the hydrocracking zone. In the hydrocracking zone, the compounds will compete with the hydrocarbons for the activity sites on the hydrocracking catalyst and thus suppress the cracking catalyst activity. Additionally, the presence of hydrogen sulfide and ammonia in the hydrocracking zone will decrease the hydrogen partial pressure. Hence, lower hydrogen partial pressure and the presence of ammonia would increase the required temperature for a fixed volume of cracking catalyst. Alternatively, to maintain a lower cracking temperature, the catalyst volume has to be increased.
Accordingly, to address these issues, some prior processes have separated ammonia and hydrogen sulfide from a hydrotreated effluent before hydrocracking. From the separated gaseous phase, hydrogen is recovered, recompressed, and recirculated back to the hydrotreating zone. This hydrogen recovery will significantly increase the heater duty, as well as the compressor capacity for this recycle gas stream.
Accordingly, it would be desirable to provide efficient and effective processes which allow for ammonia and hydrogen sulfide to be separated from the hydrotreated effluent. It would further be desirable to have such processes that did not require such a large compressor capacity.